HF and VHF antenna proximity

[GILow]

Good to know Jim, but I’m still worried by the way Ann glows in the dark.

[Jim Cate]

Originally Posted by GILow

Good to know Jim, but I’m still worried by the way Ann glows in the dark.

Did you mean "glower"?

Jim

[GILow]

I had her down as more of an ice-beam-stare type to be honest.

[waterman46]

Originally Posted by GILow

Hi all,

Can anyone tell me if there is a rule of thumb about the proximity of a VHF antenna and a HF antenna?

I’ve got a spot I really like for the spare VHF whip, but it would be reasonably close to the HF backstay antenna. (About 1 meter apart). HF is a 150 watt transmitter, VHF is 25 Watts, whip is 1.8 meters.

Matt

Since the VHF antenna length is a very small part of a quarter-wave at HF, the VHF antenna should not pick up enough HF energy to damage the VHF receiver. That is the only remotely possible area of concern. And yes the polarity will be different as well.

What I would do is test the reception of VHF weather channel while transmitting on HF at the highest frequency you anticipate using. Start with a low power of HF and work up to higher power if VHF reception does not suffer. If there is but little effect, you are really OK.

If you hear a big buzz or other unfriendly noises in the VHF in this test, you could consider using a high-pass filter on your VHF radio. You would need a filter that rejects frequencies lower than 30MHz. Be careful you don't get the type of filter used in TV systems - they are 70ohms impedance and you need a 50 ohm filter. You would need to get appropriate adapters if the connectors on the filter are not compatible with your VHF.

Now you got me thinking, and I'll try my test idea on my boat tomorrow!

[GILow]

Thanks waterman46, I won’t get a chance to perform the test for a week or more so I’ll be interested to hear of your results in the interim.

[waterman46]

Correction to my earlier post: the type of filter you would need to use is a bandpass filter with passband 156 to 162MHz. I searched on web but couldn't find any 50ohm filters that would do the trick, for a reasonable price. Highpass filters are made for TV receiver systems but they are 75ohm impedance and not made to handle 25Watt VHF power.

Interference Test:
My VHF audio on weather channel 4 went completely silent when I transmit 100W (nominal) CW on 7MHz and 17MHz ham bands. On 3.5, 10, 14 and 21 MHz ham bands, there was no effect on the VHF audio. But the VHF survived with no loss of sensitivity (which was fully expected) even after the 100W CW transmit tests on all those bands.

I have the usual antenna configuration: masthead VHF 6dB gain antenna located about 5 feet above the upper insulator of a backstay HF longwire antenna.

I suspect the interference is caused by imperfect grounding of the HF rig rather than RF pickup by the VHF antenna. I believe we have a fairly decent ground setup - autotuner is grounded to a metal tank, lifelines, engine/prop/rudder, a thru-hull bonding plate and about 1m^2 copper plate inside the hull below the waterline, all of that less than 2m from the tuner, which is directly below the backstay. The VHF radio sits within 2 feet of the HF rig and maybe the ground on the VHF antenna cable is picking up stray RF inside the boat, from the HF.

[GILow]

Thank you for those tests. I’ll have to go back to my text books to interpret the results for my system.

I seem to remember bandpass filters could be made by the home hobbyist?

[Wotname]

Originally Posted by GILow

Thank you for those tests. I’ll have to go back to my text books to interpret the results for my system.

I seem to remember bandpass filters could be made by the home hobbyist?

Remember that just having a slightly different HF & VHF antenna elements and a different spacing will change the results that waterman46 observed.

If you have the time to make a filter that most likely isn't needed, here is some bedtime reading https://www.scribd.com/doc/28332883/...buted-elements

[Jim Cate]

Matt, my friend, you have more important things to worry about...

Jim

[GILow]

Originally Posted by Jim Cate

Matt, my friend, you have more important things to worry about...

Jim

I dunno, boat decks and standing rigging are not as important has having LOTS of antennas sticking up in the air.

[GILow]

Originally Posted by Wotname

Remember that just having a slightly different HF & VHF antenna elements and a different spacing will change the results that waterman46 observed.

If you have the time to make a filter that most likely isn't needed, here is some bedtime reading https://www.scribd.com/doc/28332883/...buted-elements

See Jim, Wottie has the right idea!

[waterman46]

Originally Posted by GILow

Thank you for those tests. I’ll have to go back to my text books to interpret the results for my system.

I seem to remember bandpass filters could be made by the home hobbyist?

I found a nice video on yahoo, I think, by a guy in Poland who made a 144 to 149 MHz bandpass, with less than 0.5dB loss. That's for the 2 meter ham band. He made it using adjustable airgap capacitors for a very high Q and low loss. Extremely clever but you need a network analyzer to tune it correctly.

[Paul Elliott]

A narrow low-loss bandpass filter at VHF is difficult, and is often done with resonant cavities and air-gap capacitors. But that's not necessary for the problem being discussed.

You don't need a sharp-cutoff filter to keep 3-30 MHz SSB signals out of a 160 MHz VHF system. A simple high-pass filter at the VHF transceiver antenna connector will work, and at these power levels fairly simple components will work well with low loss.

But I don't think any filters are necessary. The VHF antenna itself generally has a built-in matching network (probably a coil at the base of the antenna, or tuned quarter-wavelength coax segments) and these will bypass most of the SSB signal. The VHF radio has it's own matching networks that will serve to further attenuate the SSB signal. My backstay SSB antenna top insulator isn't right at the masthead, and probably not as close as in the OP's case, but I've never noticed any interference and I've used both SSB and VHF simultaneously at times. The frequency difference is so great that no special steps should be required.

[GILow]

OK, I will try to relax about this one. It does sound like I am overthinking things again.

[senormechanico]

Originally Posted by teccs

Hi All,


The technical answer to antenna spacing or spacing to a metal object is: Minimum spacing should be 1/4 wavelength at the lowest frequency. For HF it would be half way up the boat. Too far to be usable. For the VHF it would be about 20 inches.


So, since we can't accommodate the HF separation, we can at least get the minimum 20 inches for the VHF.


The quarter wavelength allows an impedance transformation from low to high which makes the antenna in questions see the nearby metal as a high impedance (high resistance which is low load). The tuning of the antenna may be affected, but it usually won't be drastic.


Near field is considered to be 2 wavelengths. Beyond that distance, tuning usually won't be affected.


Wavelength is calculated as the speed of light divided by the frequency.


Tim Chapman
Owner
arrowantennas.com

OT, Am I correct in my calculation that a 2.4 gHz Wi Fi antenna on opposite sides of the mast (about 9 inches apart) from a VHF antenna is not going to be affected?
What about the VHF antenna?



Thanks,
Steve